Code 26 is an open in the field winding on the drive motor.
Start with ensuring the battery is good,
Drive fuse is not blown,
Fuses (4) on the system card,
Motor cables and connections are good,
Perform a continuity test from S1 to S2 (cables on power card, disconnect cables from Power card)
PC contact pulls in.

During self test there is a green LED on the power card that will flicker on this is normal, if it stays illuminated it has gone into an over current situation. Experience tells me this is usually a faulty card.

Remove the power card and test the 2 transistors (H bridge), each of these contain 2 transistors. During operation,1 in each is powered on and depending on direction, 1 of the others is (partially) on. Test according to the manual and do a comparison check with both modules. * replace in pairs if faulty.

If both test fine, I would trial and error replace the power card at this point it is the likely cause.
If 1 has failed, I would replace them and reinstall the original power card depending on my analysis of the cause of the failure.

If the transistor has gone open, low % of power card failure,
If the transistor has shorted, higher % of failure,

A possible cause of transistor failure is an internal failure of the power card where it overdrives the input to the transistor B1/G1 (+10v) and burns them out.